Fill Up

A slang term used when two pair or a set hits a card to make a full house.

Explained

When a player has two pair or a set on the flop and are facing a flush draw, they will say something like, "I have two pair, even if he hits the flush I still have four outs to fill up." This means that he has four cards that will improve his hand from the present two pair to a full house.

Example

Let's say you're deep in a tournament and get dealt 5s6s in the BB. The blinds are 200/400. Action folds to the player on the button who makes it an even 1000 to go. When it folds to you, you decide to call and take a flop, which comes down 6h-5h-Kd. You then check-call and the turn brings the Qh. This is a pretty scary board for your hand, so you once again check-call your opponent's bet. However, when the river comes down the 6d, you have now 'filled' up, so you move all-in and get paid when your opponent snapcalls with AhKh (the nutflush).